docs(6207): the remaining migrations state their rationale in terms of the data

Record, milestone, phase and plan citations go from every migration header;
the "end before start" fix (0043) and the suppressed-count column (0095)
state the condition they prevent rather than the incident. 0053 and 0055
pointed at design specs under docs/superpowers that are not in the repo;
those pointers are removed.

Kept: alembic's generated "Create Date:" headers (template metadata, not
rationale), 0121's ARM_BORN justification beside the constant, and 0051's
illustrative "update task 42" format example.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

alembic/versions/0103_retrieval_tuning_events.py

@@ -1,16 +1,12 @@
-"""retrieval_tuning_events — why a floor is where it is (#4102)
+"""retrieval_tuning_events — why a floor is where it is
 
 Revision ID: 0103
 Revises: 0102
 Create Date: 2026-09-17
 
-Milestone 416 stops shipping similarity thresholds as values somebody has to
-defend, and hands the adjustment to the model that reads the surface's own
-telemetry. The operator's decision:
-
-    "the floor should be chosen and adjusted by the model using it… the user
-    should be able to touch it but the model should be the thing handling it 9
-    times out of 10."
+Similarity thresholds are not values somebody has to defend: the model that
+reads the surface's own telemetry adjusts them, and the operator can still
+touch them.
 
 The number itself already has a home — the generic settings table. What has no
 home is the ARGUMENT, and once the values move on their own the argument is the